Method for generating downlink frame, and method for searching cell

ABSTRACT

The present invention relates to a method of generating a downlink frame. The method of generating the downlink frame includes generating a first sequence and a second sequence for identifying cell groups; generating a first scrambling sequence and a second scrambling sequence that are one-to-two mapped to the sequence number of the primary synchronization signal; scrambling the first sequence with the first scrambling sequence and scrambling the second sequence with the second scrambling sequence; and generating a secondary synchronization signal including the scrambled first sequence and second sequence and mapping the secondary synchronization signal to a frequency domain.

TECHNICAL FIELD

The present invention relates to a method of generating a downlink frameand a method of searching a cell, and more particularly, to a method ofgenerating a downlink frame in a cellular system based on an orthogonalfrequency division multiplexing (OFDM) scheme and a method of searchinga cell by using the downlink frame.

This work was supported by the IT R&D program of MIC/IITA[2005-S-404-13, Development of Radio Wireless Transmission Technologyfor 3G Evolution].

BACKGROUND ART

In 3^(rd) Generation Long Term Evolution (3G LTE), a total of 504 cellidentifiers are defined, and the 504 cell identifiers are divided into168 cell identifier groups. Therefore, three cell identifiers exist ineach cell identifier group. A downlink frame for 3G LTE includes twosynchronization channels, that is, a primary synchronization channel anda secondary synchronization channel. The primary synchronization channelprovides 5 msec-timing and information on three cell identifiers in thecell identifier group to a terminal. Accordingly, three primarysynchronization channel sequences are used in the LTE system, and theprimary synchronization channel sequences transmitted in a cell areequal to each other at each location of primary synchronization channelsymbols. Since the secondary synchronization channel providesinformation on the cell identifier group and 10 msec-frame timing to theterminal, the secondary synchronization channel sequences transmittedfrom the location of two secondary synchronization channel symbols inthe 10 msec-frame are different from each other. Therefore, the numberof secondary synchronization channel sequences becomes 336 (=168*2).

FIG. 1 is a view illustrating a method of generating a secondarysynchronization channel symbol according to the conventional art. Asshown in FIG. 1, the secondary synchronization channel symbol accordingto the conventional art is an OFDM signal in which two short binarysequences are combined to each other in a frequency domain, therebyproviding cell group information and frame boundary information to theterminal. The length of the binary sequences is 31, and the number ofbinary sequences is also 31. Ultimately, the number of sequences (62 inlength) of the secondary synchronization symbol that can be generated asa pair of binary sequences is 961 in total, and only 336 sequences ofthese are used.

In the 3G LTE technique, four short binary sequences are used during a10 msec-frame interval. At this time, the four short binary sequencesmay be different from one another or may be equal to one another, andfour short binary sequences are provided to every cell by the cell groupidentifier. It can use at least one same sequence or two same sequencesamong four short binary sequences between adjacent cells, particularlybetween adjacent cells when 10 msec synchronization of two cells matchwith each other. In this case, interference between adjacent cellslargely operates during a cell search process in the terminal, andperformance can be significantly deteriorated.

DETAILED DESCRIPTION

[Technical Problem]

The present invention has been made in an effort to provide a method ofgenerating a downlink frame that can reduce interference betweenadjacent cells and a method of efficiently searching a cell by receivingthe downlink frame.

[Technical Solution]

An exemplary embodiment of the present invention provides a method ofgenerating a downlink frame. The method includes: generating a firstsequence and a second sequence for identifying cell groups; generating afirst scrambling sequence and a second scrambling sequence that areone-to-two mapped to the sequence number of the primary synchronizationsignal; scrambling the first sequence with the first scrambling sequenceand scrambling the second sequence with the second scrambling sequence;and generating a secondary synchronization signal including thescrambled first sequence and second sequence and mapping the secondarysynchronization signal to a frequency domain.

Another embodiment of the present invention provides an apparatus forgenerating a downlink frame. The apparatus includes: a sequencegenerator that generates a first sequence and a second sequence foridentifying cell groups and generates a first scrambling sequence and asecond scrambling sequence that are one-to-two mapped to the sequencenumber of the primary synchronization signal; and a synchronizationsignal generator that, after scrambling the first sequence with thefirst scrambling sequence and scrambling the second sequence with thesecond scrambling sequence, generates a secondary synchronization signalincluding the scrambled first sequence and second sequence.

Yet another embodiment of the present invention provides a method ofsearching a cell. The method includes: receiving a downlink frameincluding a primary synchronization signal and secondary synchronizationsignal; and determining a cell group to which a terminal belongs, byusing the secondary synchronization signal. In this case, the secondarysynchronization signal is generated by alternately allocating a firstsequence scrambled with a first scrambling sequence and a secondsequence scrambled with a second scrambling sequence to a plurality ofsub-carriers, and the first scrambling sequence and the secondscrambling sequence are two-to-one mapped to the sequence of the primarysynchronization signal.

Yet another embodiment of the present invention provides an apparatusfor searching a cell in a terminal. The apparatus includes: afirst-stage searcher that acquires a sequence number allocated to aprimary synchronization signal; and a second-stage searcher thatidentifies a cell group to which the terminal belongs, from thesecondary synchronization signal. In this case, the secondarysynchronization signal is generated by alternately allocating a firstsequence scrambled with a first scrambling sequence and a secondsequence scrambled with a second scrambling sequence to a plurality ofsub-carriers, and the first scrambling sequence and the secondscrambling sequence are two-to-one mapped to the sequence of the primarysynchronization signal.

Yet another embodiment of the present invention provides a recordingmedium for recording a frame generating program. The frame generatingprogram includes: a function for generating a first sequence and asecond sequence for identifying cell groups; a function for generating afirst scrambling sequence and a second scrambling sequence that areone-to-two mapped to the sequence number of the primary synchronizationsignal; a function for scrambling the first sequence with the firstscrambling sequence and scrambling the second sequence with the secondscrambling sequence; and a function for generating a secondarysynchronization signal including the scrambled first sequence and secondsequence and for mapping the secondary synchronization signal to afrequency domain.

Advantageous Effects

According to the above-described present invention, interference betweensectors is reduced by scrambling two sequences to be allocated to asecondary synchronization channel with different scrambling code,thereby improving cell searching performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a method of generating a secondarysynchronization channel symbol according to the conventional art.

FIG. 2 is a view illustrating a downlink frame of an OFDM systemaccording to an exemplary embodiment of the present invention.

FIG. 3 is a block diagram of a downlink frame generating apparatusaccording to an exemplary embodiment of the present invention.

FIG. 4 is a flowchart of a downlink frame generating method according toan exemplary embodiment of the present invention.

FIG. 5 is a view illustrating an m-sequence generator.

FIG. 6 is a view illustrating a Gold sequence generator.

FIG. 7 is a view illustrating a sequence generator and a scramblingsequence generator.

FIG. 8 is a view illustrating the first secondary synchronization signalgenerating method according to an exemplary embodiment of the presentinvention.

FIG. 9 is a view illustrating the second secondary synchronizationsignal generating method according to an exemplary embodiment of thepresent invention.

FIG. 10 is a view illustrating the third secondary synchronizationsignal generating method according to an exemplary embodiment of thepresent invention.

FIG. 11 is a view illustrating the fourth secondary synchronizationsignal generating method according to an exemplary embodiment of thepresent invention.

FIG. 12 is a view illustrating the fifth secondary synchronizationsignal generating method according to an exemplary embodiment of thepresent invention.

FIG. 13 is a block diagram of a terminal according to an exemplaryembodiment of the present invention.

FIG. 14 is a block diagram of a second-stage searcher according to anexemplary embodiment of the present invention.

FIG. 15 is a flowchart illustrating a cell searching method according toan exemplary embodiment of the present invention.

BEST MODE

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

Throughout this specification, unless explicitly described to thecontrary, the word “comprise” and variations such as “comprises” or“comprising” will be understood to imply the inclusion of statedelements but not the exclusion of any other elements. In addition, theterms “-or”, “-er”, or the like described in the specification representa unit for processing at least one function and operation, which can beimplemented by hardware components or software components andcombinations thereof.

First, a configuration of a downlink frame and a synchronization channelin an OFDM system according to an exemplary embodiment of the presentinvention will be described with reference to FIG. 2.

FIG. 2 is a view illustrating a downlink frame of an OFDM systemaccording to an exemplary embodiment of the present invention. In FIG.2, the horizontal axis represents a time axis, and the vertical axisrepresents a frequency axis or a sub-carrier axis.

As shown in FIG. 2, according to the exemplary embodiment of the presentinvention, one downlink frame 110 has a time interval of 10 msec andincludes ten sub-frames 120. Moreover, one sub-frame 120 has a timeinterval of 1 msec and includes two slots 130, and one slot 130 includesseven OFDM symbols.

As shown in FIG. 2, according to the exemplary embodiment of the presentinvention, one downlink frame 110 has two synchronization intervals 140in total including a synchronization interval in slot No. 0 and slot No.10, respectively. However, it is not necessarily limited thereto. Thatis, one downlink frame 110 may include the synchronization interval inany slot, and may include one synchronization interval or threesynchronization intervals or more. Furthermore, each slot includes apilot interval.

The synchronization interval according to the exemplary embodiment ofthe present invention includes a primary synchronization channel symboland a secondary synchronization channel symbol that are disposed so asto be adjacent to each other in view of time. As shown in FIG. 2, theprimary synchronization channel symbol is located at the end of the slotand the secondary synchronization channel symbol is located right aheadof the primary synchronization channel.

In an mobile communication system according to the exemplary embodimentof the present invention, a plurality of cells are grouped into aplurality of cell groups, and each of the cell groups includes at leasttwo cells.

The primary synchronization channel includes information for identifyingsymbol synchronization and frequency synchronization and cell identifierinformation for identifying the cells in the cell groups, and thesecondary synchronization channel includes information for identifyingthe cell group information and the frame synchronization.

A downlink frame generating apparatus according to the exemplaryembodiment of the present invention will now be described with referenceto FIG. 3. FIG. 3 is a block diagram of the downlink frame generatingapparatus according to the exemplary embodiment of the presentinvention.

As shown in FIG. 3, the downlink frame generating apparatus according tothe exemplary embodiment of the present invention includes a sequencegenerator 310, a synchronization signal generator 320, a frequencymapper 330, and an OFDM transmitter 340.

The sequence generator 310 generates a sequence for identifying cellgroups and frame boundaries and a scrambling sequence, and transmitsthem to the synchronization signal generator 320. The scramblingsequence is for scrambling the sequence.

The synchronization signal generator 320 generates a secondarysynchronization signal by using the sequence and the scramblingsequence.

The frequency mapper 330 maps transmission data to the time andfrequency domains by using the synchronization signal generated in thesynchronization signal generator 320, and frame control information andtransmission traffic data transmitted from the outside, therebygenerating the downlink frame.

The OFDM transmitter 340 receives the downlink frame from the frequencymapper 330 and transmits the downlink frame through a transmissionantenna.

Referring to FIG. 4 through FIG. 12, a downlink frame generating methodaccording to the exemplary embodiment of the present invention will nowbe described. FIG. 4 is a flowchart of the downlink frame generatingmethod according to the exemplary embodiment of the present invention.

As shown in FIG. 4, the sequence generator 310 generates a plurality ofsequences and a plurality of scrambling sequences and transmits them tothe synchronization signal generator 320 (S410).

First, a method of generating the sequences will be described withreference to FIG. 5. The sequence is a cyclic shift of an m-sequencethat has a length of 31. A primitive polynomial can make the m-sequencethat has a length of 31, and six primitive polynomials are in existence,as indicated in Equation 1.P0:x⁵+x²+1P1:x⁵+x³+1P2:x⁵+x³+x²+x+1P3:x⁵+x⁴+x²+x+1P4:x⁵+x⁴+x³+x+1P5:x⁵+x⁴+x³+x²+1  (Equation 1)

FIG. 5 is a view illustrating an m-sequence generator. As shown in FIG.5, the m-sequence generator includes a plurality of delay operators 510,520, 530, 540, and 550, and an exclusive OR operator 560. In FIG. 5, g₀,g₁, g₂, g₃, g₄, and g₅ represent coefficients in the primitivepolynomial of Equation 1. The coefficients g₀ and g₅ always have a valueof 1, and the remaining coefficients g₁, g₂, g₃, and g₄ have a value of0 or 1 according to the primitive polynomial. For example, when theprimitive polynomial is x⁵+x²+1, the coefficients g₁, g₃, and g₄ have avalue of 0, and the coefficient g₂ has a value of 1. When the value ofthe coefficient is 1, the output of a corresponding delay operator isconnected to the output of the exclusive OR operator 560. Meanwhile,when the value of the coefficient is 0, the output of the correspondingdelay operator is not connected to the output of the exclusive ORoperator 560.

The downlink frame generating method according to the exemplaryembodiment of the present invention uses the cyclic shift sequence ofthe m-sequence, which is defined by the primitive polynomial of x⁵+x²+1,as a sequence. That is, assuming that c⁰={c⁰ 0, c⁰ 1, c⁰ 2, . . . , c⁰30} is referred to as the m-sequence generated by counting any value inthe sequence generator having the primitive polygonal of x⁵+x²+1, 31m-sequences may be generated as indicated in Equation 2.

$\begin{matrix}{\mspace{20mu}{c^{0} = \left\{ {{c^{0}0},{c^{0}1},{c^{0}2},\ldots\mspace{14mu},{c^{0}30}} \right\}}} & \left( {{Equation}\mspace{14mu} 2} \right) \\{c^{1} = {\left\{ {{c^{1} 0},{c^{1} 1},{c^{1} 2},\ldots\mspace{14mu},{c^{1} 30}} \right\} = \left\{ {{c^{0} 1},{c^{0}2},{c^{0}3},\ldots\mspace{14mu},{c^{0}0}} \right\}}} & \; \\{c^{2} = {\left\{ {{c^{2}0},{c^{2}1},{c^{2}2},\ldots\mspace{14mu},{c^{2}30}} \right\} = \left\{ {{c^{0}2},{c^{0}3},{c^{0}4},\ldots\mspace{14mu},{c^{0}1}} \right\}}} & \; \\{\mspace{20mu}\ldots} & \; \\{c^{30} = {\left\{ {{c^{30}0},{c^{30}1},{c^{30}2},\ldots\mspace{14mu},{c^{30}30}} \right\} = \left\{ {{c^{0}30},{c^{0}0},{c^{0}1},\ldots\mspace{14mu},{c^{0}29}} \right\}}} & \;\end{matrix}$

In Equation 2, the value of each element of the m-sequence is 1 or −1.

Next, a method of generating a scrambling sequence will be describedwith reference to FIG. 6 and FIG. 7.

A Gold sequence having a good correlation characteristic may be used asthe scrambling sequence. The Gold sequence is formed by combining two msequences having the same length.

Preferred pairs refer to pairs of m-sequences that are capable ofgenerating the Gold sequence among the m-sequences defined by the sixprimitive polynomials in Equation 1. That is, the preferred pairs arecapable of generating the Gold sequence that has a length of 31, and thenumber of preferred pairs is 12, for example [P0 P2], [P0 P3], [P0P4],[P0 P5], [P1 P2], [P1 P3], [P1 P4], [P1 P5], [P2 P3], [P2 P4], [P3 P5],and [P4 P5]. FIG. 6 is a view illustrating a Gold sequence generator.The Gold sequence generator shown in FIG. 6 generates the Gold sequenceby using the preferred pair [P1 P2].

The Gold sequence may be generated by using one of the remainingpreferred pairs except the preferred pair including the polynomial,which is used for generating the sequence, among the 12 Preferred pairs.In the downlink frame generating method according to the exemplaryembodiment of the present invention, since the polynomial x⁵+x²+1 isused as a polynomial of the sequence, the Gold sequence is generated byone of the preferred pairs [P1 P2], [P1 P3], [P1 P4], [P1 P5], [P2 P3],[P2 P4], [P3 P5], and [P4 P5]. The number of Gold sequences generated byselecting one of 8 preferred pairs is 33, which is two more than thesequence length. In 33 Gold sequences generated from the selectedpreferred pair, any sequence may be used as the scrambling sequence.

When one of the preferred pairs including the polynomial used forgenerating the sequence, that is, [P0 P2], [P0 P3], [P0 P4], and [P0P5], is selected as a preferred pair for generating the Gold sequence,the remaining Gold sequences, which exclude the sequence from (N+2) Goldsequences (where N is 31 as a length of the Gold sequence) to begenerated, should be used as a scrambling sequence. That is, 33 Goldsequences are generated by one of the preferred pairs [P0 P2], [P0 P3],[P0 P4], and [P0 P5] including the polynomial used for generating thesequence, and 33 Gold sequences include the sequence. Accordingly, itshould use the remaining Gold sequences except the sequence as ascrambling sequence.

With respect to the scrambling sequence, the m-sequence that is the samelength as the sequence but is different from the sequence in theprimitive polynomial and the cyclic shift sequence of the m-sequence maybe used. That is, as the scrambling sequence of the present invention,the m-sequence that is generated by using one of other polynomialsexcept the polynomial x⁵+x²+1 in the polynomials expressed by Equation 1and the cyclic shift sequence of the m-sequence is used. FIG. 7 is aview illustrating a sequence generator and a scrambling sequencegenerator. As shown in FIG. 7, when the m-sequence and the cyclic shiftsequence of the m-sequence are used as the scrambling sequence, thescrambling sequence may be generated by varying only a connection partof the sequence generator having the same structure as the sequence.Accordingly, this has the merit of reducing complexity of a terminal.The synchronization signal generator 320 generates the secondarysynchronization signal by using the plurality of sequences and theplurality of scrambling sequences received from the sequence generator310 (S420). The exemplary embodiment of the present invention describesthe case where one frame includes two secondary synchronizationchannels, but is not limited thereto.

Five kinds of secondary synchronization signal generating methodsaccording to the exemplary embodiment of the present invention will bedescribed with reference to FIG. 8 through FIG. 12.

FIG. 8 is a view illustrating a first secondary synchronization signalgenerating method according to the exemplary embodiment of the presentinvention. As shown in FIG. 8, according to the first secondarysynchronization signal generating method of the exemplary embodiment ofthe present invention, a first sequence c^(h) ⁰ ^((g)) (n) is scrambledwith a first scrambling sequence s^(p), and the scrambled first sequenceis allocated to even-numbered sub-carriers of the secondarysynchronization channel in the sub-frame No. 0. Furthermore, in thefirst secondary synchronization signal generating method, a secondsequence c^(h) ¹ ^((g)) (n) is scrambled with a second scramblingsequence s^(q) that is different from the first scrambling sequences^(p), and the scrambled second sequence is allocated to odd-numberedsub-carriers of the secondary synchronization channel in the sub-frameNo. 0. In addition, according to the first secondary synchronizationsignal generating method of the exemplary embodiment of the presentinvention, a third sequence c^(h) ² ^((g)) (n) is scrambled with a thirdscrambling sequence, and the scrambled third sequence is allocated toeven-numbered sub-carriers of the secondary synchronization channel inthe sub-frame No. 5. Furthermore, in the first secondary synchronizationsignal generating method, a fourth sequence c^(h) ³ ^((g)) (n) isscrambled with a fourth scrambling sequence that is different from thethird scrambling sequence, and the scrambled fourth sequence isallocated to odd-numbered sub-carriers of the secondary synchronizationchannel in the sub-frame No. 5.

That is, according to the first secondary synchronization signalgenerating method, each element of the first sequence c^(h) ⁰ ^((g)) (n)is multiplied by a corresponding element of the first scramblingsequence s^(p), and the product is allocated to even-numberedsub-carriers of the secondary synchronization channel in the sub-frameNo. 0. Moreover, each element of the second sequence c^(h) ¹ ^((g)) (n)is multiplied by a corresponding element of the second scramblingsequence s^(q), and the product is allocated to odd-numberedsub-carriers of the secondary synchronization channel in the sub-frameNo. 0. In addition, according to the first secondary synchronizationsignal generating method, each element of the third sequence c^(h) ²^((g)) (n) is multiplied by a corresponding element of the thirdscrambling sequence, and the product is allocated to even-numberedsub-carriers of the secondary synchronization channel in the sub-frameNo. 5. Moreover, each element of the fourth sequence c^(h) ³ ^((g)) (n)is multiplied by a corresponding element of the fourth scramblingsequence, and the product is allocated to odd-numbered sub-carriers ofthe secondary synchronization channel in the sub-frame No. 5.

The first scrambling sequence is expressed as s^(p)={s^(p) 0, s^(p) 1,s^(p) 2, . . . , s^(p) 30}, and the second scrambling sequence isexpressed as s^(q)={s^(q) 0, s^(q) 1, s^(q) 2, . . . , s^(q) 30}. Here,p and q represent a scrambling sequence number, respectively.

The same scrambling sequence is used in the secondary synchronizationchannel of the sub-frame No. 0 and the secondary synchronization channelof the sub-frame No. 5 within one frame, or a different scramblingsequence is used in the secondary synchronization channel of thesub-frame No. 0 and the secondary synchronization channel of thesub-frame No. 5 within one frame. FIG. 8 illustrates the case where thesame scrambling sequence is used. That is, in FIG. 8, the firstscrambling sequence and the third scrambling sequence are the same, andthe second scrambling sequence and the fourth scrambling sequence arethe same.

In the first secondary synchronization signal generating methodaccording to the exemplary embodiment of the present invention, thescrambling sequence numbers p and q are one-to-two mapped to a sequencenumber, which is allocated to the primary synchronization channel. Thatis, as described above, three primary synchronization channel sequencesare defined in the system of current 3G LTE. Therefore, according to thefirst secondary synchronization signal generating method of theexemplary embodiment of the present invention, two scrambling sequencesof the secondary synchronization channel are two-to-one mapped to theprimary synchronization signal, as indicated in Equation 3.

$\begin{matrix}{\begin{matrix}\begin{matrix}{{{Sequence}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{primary}\mspace{14mu}{synchronization}}\mspace{14mu}} \\{{channel}\text{:}\mspace{14mu}{Scrambling}\mspace{14mu}{sequence}\mspace{14mu}{numbers}\mspace{11mu} p\mspace{14mu}{and}}\end{matrix} \\{q\mspace{14mu}{of}\mspace{14mu}{secondary}\mspace{14mu}{synchronization}\mspace{14mu}{channel}}\end{matrix}\mspace{14mu}\begin{matrix}{0:} & {0,} & 1 \\{1:} & {2,} & 3 \\{2\;:} & {4,} & 5\end{matrix}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

When the scrambling sequence is given with a cyclic shifted sequence ofone m-sequence, p and q correspond to a cyclic shift index,respectively.

When the scrambling sequence numbers are one-to-two mapped to thesequence number that is allocated to the primary synchronizationchannel, the sequence number of the primary synchronization channel isdetected at a first cell searching stage of a terminal, thereby beingcapable of scrambling with the scrambling sequence number of thesecondary synchronization channel corresponding to the sequence numberof the primary synchronization channel, which is detected at the firstcell searching stage.

FIG. 9 is a view illustrating a second secondary synchronization signalgenerating method according to the exemplary embodiment of the presentinvention.

According to the second secondary synchronization signal generatingmethod according to the exemplary embodiment of the present invention, afirst sequence and a second sequence are scrambled with the samescrambling sequence, respectively, and the scrambled first and secondsequences are alternately allocated to the sub-carrier of the secondarysynchronization channel.

The second secondary synchronization signal generating method has themerit that the number of scrambling sequences is reduced by half ascompared with the first secondary synchronization signal generatingmethod. In addition, as a scrambling sequence, the Gold sequence, them-sequence, the cyclic shift sequence of the Gold sequence, or thecyclic shift sequence of the m-sequence can be used. According to thesecond secondary synchronization signal generation method, as indicatedin Equation 4, the scrambling sequence numbers are one-to-one mapped tothe sequence number allocated to the primary synchronization channel.

$\begin{matrix}{\begin{matrix}{{{Sequence}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{primary}}\mspace{14mu}} \\{{{synchronization}\mspace{14mu}{channel}\text{:}}\mspace{14mu}} \\{{{Scrambling}\mspace{14mu}{sequence}\mspace{14mu}{numbers}\mspace{14mu} p\mspace{14mu}{and}}\mspace{14mu}} \\{q\mspace{14mu}{of}\mspace{14mu}{secondary}\mspace{14mu}{synchronization}\mspace{14mu}{channel}}\end{matrix}\mspace{14mu}\begin{matrix}{0:} & {0,} & 0 \\{1:} & {1,} & 1 \\{2:} & {2,} & 2\end{matrix}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

FIG. 10 is a view illustrating a third secondary synchronization signalgenerating method according to the exemplary embodiment of the presentinvention. According to the third secondary synchronization signalgenerating method of the exemplary embodiment of the present invention,the first sequence and the second sequence are scrambled with onelong-scrambling sequence, and the scrambled first and second sequencesare alternately allocated to the secondary synchronization channel.

The length of the one long-scrambling sequence used in the thirdsynchronization signal generating method is the sum of the length of thefirst sequence, the length of the second sequence, and the length of aDC sub-carrier component. That is, the first sequence and the secondsequence have a length of 31, respectively, while the scramblingsequence used in the third synchronization signal generating method hasa length of 63.

According to the third secondary synchronization signal generatingmethod, a Gold sequence having the length of 63, an m-sequence havingthe length of 63, a shifted sequence of the Gold sequence having thelength of 63, or a shifted sequence of the m-sequence having the lengthof 63 are used as a scrambling sequence. Furthermore, the scramblingsequence is also mapped to the DC sub-carrier. That is, the thirdsecondary synchronization signal generating method is characterized bythe fact that the sequence is not mapped to the DC sub-carrier, but thescrambling sequence is mapped to the DC sub-carrier. However, an elementof the scrambling sequence mapped to the DC sub-carrier may not beactually transmitted. According to the third secondary synchronizationsignal generating method, the scrambling sequence number is one-to-onemapped to the sequence number allocated to the primary synchronizationchannel.

FIG. 11 is a view illustrating a fourth secondary synchronization signalgenerating method according to the exemplary embodiment of the presentinvention. According to the fourth secondary synchronization signalgenerating method, the first sequence is allocated to even-numberedsub-carriers of the secondary synchronization channel, the secondsequence is scrambled with the scrambling sequence, and the scrambledsecond sequence is allocated to odd-numbered sub-carriers of thesecondary synchronization channel. That is, the fourth secondarysynchronization signal generating method relates to a method of applyingthe scrambling process to only the second sequence.

The length of the scrambling sequence used in the fourth synchronizationsignal generating method is the same as that of the second sequence. Inaddition, the scrambling sequence number, which is the same as the firstsequence number, may be used. Furthermore, a Gold sequence, anm-sequence, a cyclic shift sequence of the Gold sequence, or a cyclicshift sequence of the m-sequence can used as a scrambling sequence.

FIG. 12 is a view illustrating a fifth synchronization signal generatingmethod according to the exemplary embodiment of the present invention.According to the fifth secondary synchronization signal generatingmethod, the first sequence is scrambled with the scrambling sequencecorresponding to the sequence number allocated to the primarysynchronization channel, and the scrambled first sequence is allocatedto even-numbered sub-carriers of the secondary synchronization channel.Moreover, the second sequence is scrambled with the scrambling sequencecorresponding to the first sequence number, and the scrambled secondsequence is allocated to odd-numbered sub-carriers of the secondarysynchronization channel.

In the fifth secondary synchronization signal generating method, a Goldsequence, an m-sequence, a cyclic shift sequence of the Gold sequence,or a cyclic shift sequence of the m-sequence can be used as a scramblingsequence. However, the scrambling sequence for scrambling the firstsequence and the second sequence should use different polynomials.

The frequency mapper 330 maps transmission data to the time andfrequency domains by using the synchronization signal and transmissiontraffic data generated in the synchronization signal generator 320,thereby generating the frame of downlink signals (S430).

The OFDM transmitter 340 receives the frame of downlink signals andtransmits the received downlink frame through the transmission antenna(S440).

Hereinafter, a method of searching cells in the terminal by using thedownlink signals generated in accordance with the exemplary embodimentof the present invention will be described with reference to FIG. 13through FIG. 15.

FIG. 13 is a block diagram of a terminal according to the exemplaryembodiment of the present invention, FIG. 14 is a block diagram of asecond-stage searcher according to the exemplary embodiment of thepresent invention, and FIG. 15 is a flowchart illustrating a cellsearching method according to the exemplary embodiment of the presentinvention.

As shown in FIG. 13, the terminal according to the exemplary embodimentof the present invention includes a receiving antenna 610, a downconverter 620, a cell searcher 630, a data channel demodulator 640, acontroller 650, and a clock generator 660. In addition, the cellsearcher 630 includes a synchronization channel band-pass filter 631, afirst-stage cell searcher 632, and a second-stage searcher 633.

The synchronization channel band-pass filter 631 receives receivedsignals S1 and S2 and filters only a synchronization channel in anentire received band. The first-stage searcher 632 receives outputsignals S3 and S4 of the synchronization channel band-pass filter 631and acquires 5 msec-timing-information S5 of the downlink signal andsequence number S6 of the primary synchronization channel. Furthermore,the second-stage searcher 633 acquires a cell group identifier S7 and 10msec-frame-boundary S8 by using the secondary synchronization channelstructure of the present invention based on the information S5 and S6received from the first-stage searcher 632. Moreover, the second-stagesearcher 633 extracts a cell identifier S9 by combining the sequencenumber S6 of the primary synchronization channel and the cell groupidentifier S7, and can transfer the extracted cell group identifier S7to the controller 650.

As shown in FIG. 14, the second-stage searcher 633 includes cyclicprefix (CP) removers 633 a, fast Fourier transform transformers (FFTs)633 b, channel estimators 633 c, channel compensators 633 d, an antennacoupler 633 e, a scrambling sequence generator 633 f, a sequencegenerator 633 g, a descrambling block 633 h, and a maximum valueselector 633 i.

As shown in FIG. 15, the CP removers 633 a receive the output signals S3and S4 of the synchronization channel band-pass filter 631 and removethe CP from the position of the primary synchronization channel symboland the secondary synchronization channel symbol based on the 5msec-timing-information S5 received from the first-stage searcher 632(S710).

The fast Fourier transformers 633 b perform fast Fourier transform onthe output signals of the CP removers 633 a and change a time domainsignal into a frequency domain signal (S720).

The channel estimators 633 c estimate the channel by using a primarysynchronization channel component S10 among the outputs of the fastFourier transformer (FFT) 633 b (S730), and the channel compensators 633d compensate channel distortion of a secondary synchronization channelcomponent among the outputs of the fast Fourier transformer (FFT) 633 bby using channel values received from the channel estimators 633 c(S740).

The antenna coupler 633 e performs diversity combination for tworeceiving antenna signals that are outputs of two channel compensators633 d (S750). The descrambling block 633 h descrambles the output of theantenna coupler 633 e by using the scrambling sequence generated by thescrambling sequence generator 633 f and the sequence generated by thesequence generator 633 g (S760).

The maximum value selector 633 i selects the maximum value among theoutputs of the descrambling block 633 h, determines the cell groupidentifier S7 and the 10 msec-frame-boundary S8 from the maximum indexvalue, and outputs the determined cell group identifier S7 and 10msec-frame-boundary S8 (S770). Moreover, the maximum value selector 633i can also analogize the cell identifier number S9 by using the sequencenumber S6 of the primary synchronization channel and the cell groupidentifier number S7 (S780).

The exemplary embodiment of the present invention is not necessarilyimplemented by only the above-described apparatus and/or method, but canbe implemented by, for example, a program that achieves the functioncorresponding to the configuration of the exemplary embodiment of thepresent invention and a recording medium in which the program isrecorded. This can be easily implemented from the above-describedexemplary embodiment of the present invention by those skilled in therelated art.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for generating a frame, the method comprising: generating afirst sequence and a second sequence for identifying cell groups;generating a first scrambling sequence and a second scrambling sequencethat are mapped to a sequence number of a primary synchronizationsignal; scrambling the first sequence with the first scrambling sequenceand scrambling the second sequence with the second scrambling sequence;generating a secondary synchronization signal including the scrambledfirst sequence and the scrambled second sequence; and mapping thesecondary synchronization signal to a frequency domain.
 2. The method ofclaim 1, wherein the mapping of the secondary synchronization signalcomprises alternately disposing the scrambled first sequence and thescrambled second sequence on a plurality of sub-carriers.
 3. The methodof claim 1, wherein the scrambling of the first sequence comprisesmultiplying each element of the first sequence by each element of thefirst scrambling sequence corresponding to each element of the firstsequence, and wherein the scrambling of the second sequence comprisesmultiplying each element of the second sequence by each element of thesecond scrambling sequence corresponding to each element of the secondsequence.
 4. The method of claim 1, wherein the first sequence and thesecond sequence are cyclic shifted sequences of an m-sequence,respectively.
 5. The method of claim 1, wherein the first scramblingsequence is an m-sequence that has different primitive polynomial fromthe first sequence or a cyclic shifted sequence of the m-sequence.
 6. Anapparatus for generating a frame, the apparatus comprising: a sequencegenerator that generates a first sequence and a second sequence foridentifying cell groups and generates a first scrambling sequence and asecond scrambling sequence that are mapped to a sequence number of aprimary synchronization signal; and a synchronization signal generatorthat, after scrambling the first sequence with the first scramblingsequence and scrambling the second sequence with the second scramblingsequence, generates a secondary synchronization signal including thescrambled first sequence and the scrambled second sequence.
 7. Theapparatus of claim 6, wherein the first sequence and the second sequenceare a cyclic shift sequence of an m-sequence, respectively.
 8. Theapparatus of claim 6, wherein the first scrambling sequence is anm-sequence that has different primitive polynomial from the firstsequence or a cyclic shifted sequence of the m-sequence.
 9. A method forsearching for a cell, the method comprising: receiving a downlink frameincluding a primary synchronization signal and a secondarysynchronization signal; and determining a cell group to which a terminalbelongs, by using a secondary synchronization signal, wherein thesecondary synchronization signal is generated by alternately allocatinga first sequence scrambled with a first scrambling sequence and a secondsequence scrambled with a second scrambling sequence to a plurality ofsub-carriers, and the first scrambling sequence and the secondscrambling sequence are mapped to a sequence number of the primarysynchronization signal.
 10. The method of claim 9, further comprisingidentifying a cell to which the terminal belonged, by using the primarysynchronization signal.
 11. The method of claim 9, wherein the firstsequence and the second sequence are a cyclic shifted sequence of anm-sequence, respectively.
 12. The method of claim 9, wherein the firstscrambling sequence is an m-sequence that has different primitivepolynomial from the first sequence or a cyclic shifted sequence of them-sequence.
 13. An apparatus for searching for a cell in a terminal,which receives a downlink frame including a primary synchronizationsignal and a secondary synchronization signal, the apparatus comprising:a first-stage searcher for acquiring a sequence number allocated to theprimary synchronization signal; and a second-stage searcher foridentifying a cell group to which the terminal belongs, from thesecondary synchronization signal, wherein the secondary synchronizationsignal is generated by alternately allocating a first sequence scrambledwith a first scrambling sequence and a second sequence scrambled with asecond scrambling sequence to a plurality of sub-carriers, and the firstscrambling sequence and the second scrambling sequence are mapped to asequence number of the primary synchronization signal.
 14. The apparatusof claim 13, wherein the first sequence and the second sequence are acyclic shift sequence of an m-sequence, respectively.
 15. The apparatusof claim 13, wherein the first scrambling sequence is an m-sequence thathas different primitive polynomial from the first sequence or a cyclicshifted sequence of the m-sequence.